In religion especially for Hinduism and Buddhism, enlightenment basically means you have reached the highest level of awareness or in other word you have gone to a place called "Nirvana". This nirvana is the ultimate place but I have a slightly different opinion than this.
Many people misunderstand the enlightenment is generic but it is not. It is very much very specific to a person depends on how he or she perceives the system that he or she is living in. Let's read this is post: what exactly is the system? to get an idea what system that I mean here.
Let's assume there is an ant living on earth. Does he know that the earth is round and rotating itself? He doesn't know but when he knows one day, it is called enlightenment but is it ultimate? No, because one day he may find out the earth is rotating around the sun and he is enlighten again. In other word, he has gone to a higher level system.
It is very similar to "Nirvana" which is defined or perceived by human. Nirvana to the ant is understanding the earth is round and rotating itself. To human's nirvana, it is something else. To you maybe is another thing. That's why enlightenment is very specific to a person.
To make it slightly confusing, one cannot be considered as enlightenment if he still can't define the milestones in the system where he came from. If he can't, he is not enlightened yet, it also means he has not "fully" understand the system yet. See this previous post: who define the milestones?
So, what is Enlightenment?